1. Field of the Invention
The present invention relates to a highly reliable data storage system, which can rescue data from a defective bit even when the defective bit is generated, and more particularly, to a data storage system which has enhanced reliability with respect to data destruction caused by reading-out of the data.
2. Description of the Related Art
A nonvolatile semiconductor memory (EEPROM) has been developed which uses electrical charges, for information storage of digital bits, injected from a channel through an insulating film to a charge storage layer by means of a tunnel current. A conductance change of a field effect transistor (MOSFET) is determined in accordance with the amount of the charges to read out information. In particular, a NAND or virtual ground type EEPROM, in which a plurality of memory cells are connected serially or in parallel to form a memory cell block, can greatly reduce the number of selection transistor gates compared to the number of memory cells, and achieve a higher density. Here, different from a DRAM, in the EEPROM, data can be read out a plurality of times without involving data destruction. However, a reading operation applies a voltage to a selected gate of an unselected EEPROM cell, and repeated reading operations cause a read disturb once which destroys data. Consequently, a problem arises that a large current cannot be conducted during the reading.